a) Field of the Invention
The present invention relates to improvements in sound reproduction devices and, in particular, in such devices having active noise compensation,
b) Description of the Related Art
Noise is one of the most severe environmental pollutants and is a stress factor to be taken seriously. Studies have shown that noise acts on the vegetative nervous system, resulting in fatigue, loss of concentration, nervousness, and irritability. Further, continuous effects of noise lead to permanent hearing loss.
In order to combat these problems, sound reproduction devices with active noise compensation based on the principle of phase-inverted sound are already known.
For this purpose, the sound wave occurring at the site of influence, e.g., the ear, is fed at this location to a filter by means of an acoustic pickup in the form of a microphone for a phase shift of 180.degree. and the phase-inverted sound is emitted via a transducer.
A noise reduction of more than 15 dB can be achieved in the low frequency range with an active noise compensation device of this kind in combination with passive hearing protection or closed headphones. A noise reduction of 10 dB is perceived subjectively as a 50% reduction in loudness.
Such headphones with active noise compensation have been commercially available for some years, e.g., under the trade name "NoiseGard.RTM." (trademark of Sennheiser electronic KG), model HDC 200 "NoiseGard.RTM. mobile". The principle of active noise compensation is also known, for example, from the following references: DE-A-95134, DE-B-305391, DE-C-71754, DE-C-71534, DE-C-655508, DE-A-3719963, DE-C-40153, DE-U-881597, EP-A-008389, GB-A-147166, GB-A-16074, GB-A-160070, GB-A-09769, GB-C-1530814, DE-A-33498, DE-A-3137747, DE-151717, EP-A-0461801, U.S. Pat. No. 4,736,431, U.S. Pat. No. 4,622,692, U.S. Pat. No. 4,494,074, U.S. Pat. No. 4,05,734, U.S. Pat. No. 4,017,797, U.S. Pat. No. 3,952,158, U.S. Pat. No. 3,637,040, U.S. Pat. No. 2,972,018 or U.S. Pat. No. 3,043,416, GB-2,187,361, U.S. Pat. No. 3,637,040, U.S. Pat. No. 4,922,542, U.S. Pat. No. 4,399,334, US-RE-260,030, and U.S. Pat. No. 1,807,225.
Finally, a high compliance headphone transducer used for an active noise compensation device is known from U.S. Pat. No. 5,181,252. In this known device, the cavity in front of the transducer is separated from the closed cavity to the rear of the transducer by the transducer membrane or diaphragm. Further, the transducer has a diaphragm which is considerably more compliant than the rear cavity volume or, in other words, the rear volume is appreciably stiffer than the stiffness of the diaphragm of the transducer. Such a ratio of diaphragm stiffness to the stiffness of the rear volume is achieved, e.g., in a transducer diaphragm formed of a polycarbonate film having a thickness of 40 .mu.m. In the noise compensation device known from U.S. Pat. No. 5,181,252, the rear volume accordingly determines the total stiffness of the arrangement comprising the transducer and rear volume. A known device of this kind has a relatively low resonance frequency and is less mechanically robust relative to environmental influences such as pressure and temperature fluctuations. As a result, the transducer is exposed to the risk of mechanical damage especially when the active noise compensation device is used under extreme environmental conditions, as is not uncommon in air traffic.